Continuous processes for the highly selective conversion of aldohexose-yielding carbohydrate to ethylene glycol

ABSTRACT

Continuous processes for making ethylene glycol form aldohexose-yielding carbohydrates are disclosed which enhance the selectivity to ethylene glycol.

CROSS REFERENCE TO RELATED PATENT APPLICATIONS

This Application is a continuation of U.S. application Ser. No.15/612,700, filed on Jun. 2, 2017, which claims the benefit of U.S.Provisional Application No. 62/345,399, filed Jun. 3, 2016, each ofwhich is incorporated by reference in its entirety.

FIELD OF THE INVENTION

This invention pertains to processes for the production of ethyleneglycol, particularly high-efficiency, continuous processes for theconversion of aldohexose-yielding carbohydrate to ethylene glycol.

BACKGROUND

Ethylene glycol is a valuable commodity chemical that has a broad rangeof uses as both a building block for other materials such aspolyethylene terephthalate (PET) and for its intrinsic properties suchas for antifreeze. Ethylene glycol demand is substantial, making it oneof the largest volume organic chemicals produced in the world. It iscurrently made by multistep processes which start with ethylene derivedfrom hydrocarbon feedstocks.

Proposals have been made to manufacture ethylene glycol from renewableresources such as carbohydrates. See, for instance, U.S. Pat. No.5,210,335; EP 2419393; U.S. Published Pat. Appl. 2012/0172633; and GreenChem., 2014, 16, 695-707. Until recently, the proposed processes formanufacturing ethylene glycol from carbohydrates have suffered fromextremely low selectivities to ethylene glycol. More recent proposalshave focused on the use of two catalysts for the conversion ofcarbohydrates to ethylene glycol. One catalyst affects a retro-aldolreaction, and the second is used for hydrogenation. Thus, for instance,an aldohexose is converted to glycolaldehyde and erythrose, and theglycolaldehyde is in turn hydrogenated to ethylene glycol. Erythrose canundergo additional retro-aldol reaction to provide two more molecules ofglycolaldehyde. Although the processes provide higher selectivity toethylene glycol, a need still remains to provide a process that would becommercially competitive with conventional processes using ethylene asthe feedstock.

Schreck, et al., in U.S. Published Pat. Appl. 2015/0329449, discloseimproved, continuous processes for the conversion of carbohydrates toethylene glycol and propylene glycol. They disclose using a reactor forthe conversion of carbohydrates to the glycols which has a first zonecomprising a retro-aldol catalyst and a second zone comprising aretro-aldol and reducing catalyst. Where the feed is an aldose,glycolaldehyde from the retro-aldol reaction is hydrogenated in thesecond zone of the reactor to ethylene glycol. They also disclose usingketose as the carbohydrate to produce propylene glycol.

Nevertheless, challenges still remain to further enhance the selectivityof the conversion of carbohydrates to glycols, especially to ethyleneglycol. These challenges are not insignificant due to the myriad ofreactions that can occur under the conditions required for theretro-aldol reaction and for the hydrogenation, including, but notlimited to, hydrogenation of the hexose to hexitol and the formation ofside products such as methane, methanol, ethanol, propanol, glycerin,1,2-butanediol, threitol, and humins. Although some side products may bemarketable, their recovery to meet merchant grade specification can becostly. Moreover, glycolaldehyde is highly reactive.

SUMMARY

By this invention, continuous processes are provided that enhance theselectivity of a retro-aldol and hydrogenation conversion ofaldohexose-yielding carbohydrate to ethylene glycol.

In accordance with the first broad aspect of this invention, a rapidheating of a carbohydrate being fed into a reaction zone can reduce theproduction of hexitol and other side products such as propylene glycol.The mechanism by which the rapid heating of the carbohydrate feedresults in the enhance selectivity of conversion to ethylene glycol isnot fully understood. Without wishing to be limited to theory, it isbelieved, in part, that at temperatures above 230° C. in the presence ofretro-aldol catalyst, the rate of the retro-aldol conversion of aldosesto glycolaldehyde is sufficiently rapid that preferentially aldose isreacted to make glycolaldehyde and intermediates that can provideethylene glycol in comparison to other reactions that can generate sideproducts or reduce selectivity of conversion to ethylene glycol.

Measuring directly the rate of heating of the carbohydrate feedcontaining the aldohexose-yielding carbohydrate is problematic due tothe speed at which the heating occurs. The problem of temperaturemeasurement is further confounded by the heat and mass transfer throughthe fluid which is being heated. The heat and mass transfer parameterswithin a given fluid will depend upon many factors including, but notlimited to, the method of heating, the temperature differential, and thephysical structure of the zone in which the heating is occurring.Moreover, the analytical techniques to measure temperatures essentiallyin all regions of the fluid are practically unavailable. Accordingly,ascertaining whether or not the rate of heating is sufficient can onlypractically be done by reference to the relative formation of certaincompounds generated in the practice of the process. Nevertheless, it isbelieved that the rate of heating is sufficient to raise the temperatureof the entire carbohydrate feed from about 170° C. to at least 230° C.in less than about 10 seconds, and more preferably in less than about 5seconds, and in some instances less than about 3 seconds, and in someother instances less than about 1 second.

The temperature range through which the carbohydrate feed is to berapidly heated in accordance with processes of this invention is frombelow 170° C. to above 230° C. In some instances, the carbohydrate feedmay be at a temperature below about 150° C., or even below about 100°C., when the rapid heating commences. In some instances it is preferredthat where the carbohydrate feed contains retro-aldol catalyst, therapid heating of the carbohydrate feed commences prior to about 100° C.

In this first broad aspect of the invention, the high-selectivity,continuous processes for converting aldohexose in an aldohexose-yieldingcarbohydrate-containing feed to ethylene glycol comprise:

-   a. continuously or intermittently passing said carbohydrate feed    into a reaction zone having an aqueous, hydrogenation medium    containing retro-aldol catalyst, hydrogen and hydrogenation    catalyst;-   b. maintaining the aqueous, hydrogenation medium in the reaction    zone at hydrogenation conditions to provide a product solution    comprising ethylene glycol, propylene glycol and hexitol, said    hydrogenation conditions comprising a temperature in the range of    between about 230° C. to 300° C., a ratio of retro-aldol catalyst to    hydrogenation catalyst, and hydrogen partial pressure that, in    combination, are sufficient to:    -   i. convert at least about 95 percent of the aldohexose-yielding        carbohydrate,    -   ii. provide a conversion efficiency of said aldohexose to        ethylene glycol of at least about 60 percent, and    -   iii. provide a mass ratio of ethylene glycol to hexitol greater        than about 10:1 in the product solution; and-   c. continuously or intermittently withdrawing product solution from    said reaction zone,-   wherein said carbohydrate feed is at least partially hydrated and is    under a pressure sufficient to maintain partial hydration; wherein    said carbohydrate feed is below a temperature of about 170° C.; and    wherein said carbohydrate feed is heated to above 230° C.    immediately prior to or in the reaction zone and the rate of heating    of the carbohydrate feed from below 170° C. to above 230° C. is    sufficient to provide a product solution having at least one of:    -   A. a mass ratio of ethylene glycol to propylene glycol from said        aldohexose of at least about 15:1, and    -   B. a mass ratio of glycerin to propylene glycol from said aldose        of less than about 0.5:1

The carbohydrate feed may be admixed with retro-aldol catalyst prior tobeing heated to a temperature above 230° C. or may be substantiallydevoid of any retro-aldol catalyst. In some aspects of the invention,the carbohydrate feed will contain retro-aldol catalyst. In theseaspects, retro-aldol reactions may occur during the heating. Because thecarbohydrate feed is introduced into the aqueous, hydrogenation mediumupon heating, any glycolaldehyde generated is promptly available forhydrogenation to ethylene glycol.

In yet a further expression of this first broad aspect of the invention,the high-selectivity, continuous processes for converting aldohexose inan aldohexose-yielding carbohydrate-containing feed to ethylene glycolcomprise:

-   a. continuously or intermittently passing said carbohydrate feed    into a reaction zone having an aqueous, hydrogenation medium    containing retro-aldol catalyst, hydrogen and hydrogenation    catalyst;-   b. maintaining the aqueous, hydrogenation medium in the reaction    zone at hydrogenation conditions to provide a product solution    comprising ethylene glycol, propylene glycol, glycerin and hexitol,    said hydrogenation conditions comprising a temperature in the range    of between about 230° C. to 300° C., a ratio of retro-aldol catalyst    to hydrogenation catalyst, and hydrogen partial pressure that, in    combination, are sufficient to convert at least about 95 percent of    the aldohexose-yielding carbohydrate; and-   c. continuously or intermittently withdrawing product solution from    said reaction zone,-   wherein said carbohydrate feed is at least partially hydrated and is    under a pressure sufficient to maintain partial hydration; wherein    said carbohydrate feed is below a temperature of about 170° C.; and    wherein said carbohydrate feed is heated at least in part by direct    heat exchange with a warmer fluid to above about 230° C. immediately    prior to or in the reaction zone and the rate of heating of the    carbohydrate feed from below 170° C. to above 230° C. is sufficient    to provide a product solution having at least one of:    -   A. a mass ratio of glycerin to propylene glycol from said aldose        of less than about 0.5:1, and    -   B. a mass ratio of ethylene glycol to hexitol greater than about        20:1 and a mass ratio of ethylene glycol to propylene glycol        from said aldohexose of at least about 15:1.

The warmer fluid for the direct heat exchange may be any suitable fluidand often comprises water. The temperature and amount of the warmerfluid are sufficient to enable the carbohydrate feed to achieve, incombination with any other heating source, a temperature of at least230° C. Often, the warmer fluid is above 230° C., and in some instancesabove 235° C. The carbohydrate feed can be introduced into a reactionzone for the hydrogenation, or, if used, a prior retro-aldol reactionzone having an essential absence of hydrogenation catalyst, and theaqueous medium contained in such reaction zone serves as the warmerfluid. Alternatively, a warmer fluid may be combined with thecarbohydrate feed prior to the combination being introduced into aretro-aldol or combination retro-aldol and hydrogenation reaction zone.

The commencement of the contact between the carbohydrate feed and theretro-aldol catalyst may occur in the reaction zone containing thehydrogenation catalyst or in a separate reaction zone. Where the contactis commenced in a separate reaction zone, all or a portion of thealdohexose can be reacted in the separate reaction zone. In someinstances, all or a portion of the aldohexose undergoes retro-aldolconversion in the reaction zone containing the hydrogenation catalyst,e.g., at least about 10, and sometimes at least about 20, mass percentto essentially all of the aldohexose in the carbohydrate feed undergoesretro-aldol conversion in the reaction zone containing the hydrogenationcatalyst. In the certain embodiments of the instantly disclosedprocesses, the retro-aldol catalyst is a homogeneous catalyst and thehydrogenation catalyst is heterogeneous. Thus, the dispersion of theretro-aldol catalyst within the region occupied by the hydrogenationcatalyst can provide glycolaldehyde and other intermediates that canprovide ethylene glycol proximate to hydrogenation sites.

The amount of hydrogenation catalyst required for a given circumstancewill depend upon the relative activity of the catalyst and the masstransfer of hydrogen and glycolaldehyde and intermediates to thecatalyst. A preferred hydrogenation catalyst is a supportednickel-containing hydrogenation catalyst, especially nickel catalystscontaining one or both of rhenium and iridium. The ratio of theretro-aldol catalyst to hydrogenation catalyst is preferably sufficientthat the production of hexitols, e.g., from the hydrogenation ofaldohexose, is minimized. However, it is preferred that thehydrogenation catalyst have a density in the reaction zone sufficient tocause hydrogenation of glycolaldehyde and other intermediates beforecompetitive reactions of glycolaldehyde and other intermediates are ableto generate products other than ethylene glycol.

Preferably carbohydrate-containing feeds that provide between about 120and 700 or 800, preferably between about 150 and 500, say, 200 to 400,grams total carbohydrate per liter of aqueous, hydrogenation medium toprovide product solutions having advantageous ratios of ethylene glycolto propylene glycol and reduced co-production of 1,2-butanediol. Withoutwishing to be limited by theory, it is believed that glycolaldehydegenerated by the retro-aldol conversion of aldohexose can form a dimeror other structure (“protected species”) that has less reactivity underthe hydrogenation conditions than does glycolaldehyde. Thus, at lowconcentrations of carbohydrate feed in an aqueous reaction media, masstransfer limitations exist that attenuate the production of protectedspecies. Consequently, a greater portion of the glycolaldehyde generatedcan enter into competitive reactions with other components in theaqueous reaction medium. Conversely, at high concentrations ofcarbohydrate feed in an aqueous reaction medium, unreacted aldoses andother carbohydrates in the carbohydrate feed would increase theprobability of contact with glycolaldehyde for competitive reactions orhydrogenation catalyst.

According to certain embodiments, the rate of the carbohydrate feed isin a range sufficient to provide a product solution having a mass ratioof 1,2-butanediol to ethylene glycol of less than about 1:30, andpreferably less than about 1:50.

Each of the elements recited herein for each of the above broad aspectsof this invention pertaining to processes for making ethylene glycol, tothe extent not recited explicitly in other broad aspects of thisinvention are optional, or preferred, elements for the other broadprocesses of this invention.

Another broad aspect of the invention pertains to aqueous producercompositions comprising (excluding catalysts and water) containingethylene glycol, propylene glycol, hexitol, glycerin and 1,2-butanediolwherein:

-   -   a. ethylene glycol is present in an amount of at least about 70        mass percent,    -   b. propylene glycol is present in a mass ratio of ethylene        glycol to propylene glycol of between about 15:1 to 35:1,    -   c. hexitol is present in a mass ratio of ethylene glycol to        hexitol of greater than about 20:1,    -   d. glycerin is present in a mass ratio of glycerin to propylene        glycol of less than 0.5:1, and    -   e. 1,2-butanediol is present in a mass ratio of 1,2-butanediol        to ethylene glycol of less than about 1:30.

With respect to the above broad aspects of this invention, thecarbohydrate feed contains an aldohexose-yielding carbohydrate. Thealdohexose-yielding carbohydrate may be an aldohexose, per se, or may bea di- or polysaccharide that upon hydrolysis yields an aldohexose.Within the broad scope of this invention, the carbohydrate feed may alsoinclude a ketose-yielding carbohydrate or other aldose-yieldingcarbohydrate. In the instances where a ketose-yielding carbohydrate orother aldose-yielding carbohydrate is contained in the carbohydratefeed, a lower mass ratio of ethylene glycol to propylene glycol willresult through the retro-aldol and hydrogenation reactions than thatfrom a feed providing essentially only aldohexose. The processes of thisinvention also contemplate the addition of propylene glycol to thereactor. Nevertheless the ethylene glycol to propylene glycol mass ratiois that attributable to the aldohexose is preferably greater than 15:1.

In some instances, the carbohydrate feed can be a melted solid, in whichcase it should remain at least partially hydrated in order to avoidcaramelization during the heating. Preferably the carbohydrate feed isprovided as an aqueous solution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a facility capable of using theprocess of this invention, according to certain embodiments.

DETAILED DESCRIPTION

All patents, published patent applications and articles referencedherein are hereby incorporated by reference in their entirety.

Definitions

As used herein, the following terms have the meanings set forth belowunless otherwise stated or clear from the context of their use.

Where ranges are used herein, the end points only of the ranges arestated so as to avoid having to set out at length and describe each andevery value included in the range. Any appropriate intermediate valueand range between the recited endpoints can be selected. By way ofexample, if a range of between 0.1 and 1.0 is recited, all intermediatevalues (e.g., 0.2, 0.3. 6.3, 0.815 and so forth) are included as are allintermediate ranges (e.g., 0.2-0.5, 0.54-0.913, and so forth).

The use of the terms “a” and “an” is intended to include one or more ofthe element described.

Admixing or admixed means the formation of a physical combination of twoor more elements which may have a uniform or non-uniform compositionthroughout and includes, but is not limited to, solid mixtures,solutions and suspensions.

Aldose means a monosaccharide that contains only a single aldehyde group(—CH═O) per molecule and having the generic chemical formulaC_(n)(H2O)_(n). Non-limiting examples of aldoses include aldohexose (allsix-carbon, aldehyde-containing sugars, including glucose, mannose,galactose, allose, altrose, idose, talose, and gulose); aldopentose (allfive-carbon aldehyde containing sugars, including xylose, lyxose,ribose, and arabinose); aldotetrose (all four-carbon, aldehydecontaining sugars, including erythrose and threose) and aldotriose (allthree-carbon aldehyde containing sugars, including glyceraldehyde).

Aldose-yielding carbohydrate means an aldose or a di- or polysaccharidethat can yield aldose upon hydrolysis. Most sugars are ring structuresunder ambient conditions and thus the aldose form occurs under theconditions of the process of this invention. Sucrose, for example, is analdose-yielding carbohydrate even though it also yields ketose uponhydrolysis.

Aqueous and aqueous solution mean that water is present but does notrequire that water be the predominant component. For purposes ofillustration and not in limitation, a solution of 90 volume percent ofethylene glycol and 10 volume percent water would be an aqueoussolution. Aqueous solutions include liquid media containing dissolved ordispersed components such as, but not in limitation, colloidalsuspensions and slurries.

Bio-sourced carbohydrate feedstock means a product that includescarbohydrates sourced, derived or synthesized from, in whole or insignificant part, to biological products or renewable agriculturalmaterials (including, but not limited to, plant, animal and marinematerials) or forestry materials.

Commencing contact means that a fluid starts a contact with a component,e.g., a medium containing a homogeneous or heterogeneous catalyst, butdoes not require that all molecules of that fluid contact the catalyst.

Compositions of aqueous solutions are determined using gaschromatography for lower boiling components, usually components having 3or fewer carbons and a normal boiling point less than about 300° C., andhigh performance liquid chromatography for higher boiling components,usually 3 or more carbons.

Conversion efficiency of aldohexose to ethylene glycol is reported inmass percent and is calculated as the mass of ethylene glycol containedin the product solution divided by the mass of aldohexose theoreticallyprovided by the carbohydrate feed and thus includes any aldohexose perse contained in the carbohydrate feed and the aldohexose theoreticallygenerated upon hydrolysis of any di- or polysaccharide contained in thecarbohydrate feed.

Hexitol means a six carbon compound having the empirical formula ofC₆H₁₄O₆ with one hydroxyl per carbon. Hexitols can have differentstereoconfigurations, e.g., sorbitol and mannitol.

High shear mixing involves providing a fluid traveling at a differentvelocity relative to an adjacent area which can be achieved throughstationary or moving mechanical means to affect a shear to promotemixing. As used herein, the components being subjected to high shearmixing may be immiscible, partially immiscible or miscible.

Hydraulic distribution means the distribution of an aqueous solution ina vessel including contact with any catalyst contained therein.

Immediately prior to means no intervening unit operation requiring aresidence time of more than one minute exists.

Intermittently means from time to time and may be at regular orirregular time intervals.

Ketose means a monosaccharide containing one ketone group per molecule.Non-limiting examples of ketoses include ketohexose (all six-carbon,ketone-containing sugars, including fructose, psicose, sorbose, andtagatose), ketopentose (all five-carbon ketone containing sugars,including xylulose and ribulose), ketotetrose (all four-carbon, ketosecontaining sugars, including erythrulose), and ketotriose (allthree-carbon ketose containing sugars, including dihydroxyacetone).

pH of an aqueous solution is determined at ambient pressure andtemperature. In determining the pH of, for example the aqueous,hydrogenation medium or the product solution, the liquid is cooled andallowed to reside at ambient pressure and temperature for 2 hours beforedetermination of the pH.

pH control agents means one or more of buffers and acids or bases.

A pressure sufficient to maintain at least partial hydration of acarbohydrate means that the pressure is sufficient to maintainsufficient water of hydration on the carbohydrate to retardcaramelization. At temperatures above the boiling point of water, thepressure is sufficient to enable the water of hydration to be retainedon the carbohydrate.

A rapid diffusional mixing is mixing where at least one of the two ormore fluids to be mixed is finely divided to facilitate mass transfer toform a substantially uniform composition. Fractal mixing is a rapiddiffusional mixing.

A reactor can be one or more vessels in series or in parallel and avessel can contain one or more zones. A reactor can be of any suitabledesign for continuous operation including, but not limited to, tanks andpipe or tubular reactor and can have, if desired, fluid mixingcapabilities. Types of reactors include, but are not limited to, laminarflow reactors, fixed bed reactors, slurry reactors, fluidized bedreactors, moving bed reactors, simulated moving bed reactors,trickle-bed reactors, bubble column and loop reactors.

Soluble means able to form a single liquid phase or to form a colloidalsuspension.

Carbohydrate Feed

The processes of this invention use a carbohydrate feed that contains analdohexose-yielding carbohydrate. In some instances, the carbohydratefeed comprises at least about 40, and preferably at least about 50, masspercent of aldohexose-yielding carbohydrate based upon totalcarbohydrate in the feed. Where product solutions containing a high massratio of ethylene glycol to propylene glycol are sought, thecarbohydrate in the feed comprises at least about 90, preferably atleast about 95 or 99, mass percent of aldohexose-yielding carbohydrate.Often the carbohydrate feed comprises a carbohydrate polymer such asstarch, cellulose, or partially hydrolyzed fractions of such polymers ormixtures of the polymers or mixtures of the polymers with partiallyhydrolyzed fractions.

Most bio-sourced carbohydrate feedstocks yield glucose upon beinghydrolyzed. The processes of this invention can be effectively used forthe conversion of glucose and glucose precursors to ethylene glycol.Glucose precursors include, but are not limited to, maltose, trehalose,cellobiose, kojibiose, nigerose, nigerose, isomaltose, β,β-trehalose,α,β-trehalose, sophorose, laminaribiose, gentiobiose, and mannobiose. Itis also acceptable to have glucose as a major portion or sole reactivecomponent of the carbohydrate feed. Of course, other aldoses can be usedin the processes of this invention. Other carbohydrate polymers andoligomers such as hemicellulose, partially hydrolyzed forms ofhemicellulose, disaccharides such as sucrose, lactulose, lactose,turanose, maltulose, palatinose, gentiobiulose, melibiose, andmelibiulose, or combinations thereof may be used. However, the nature ofthese may result in variable mixtures of ethylene glycol and propyleneglycol.

The carbohydrate feed can be solid or in a liquid suspension ordissolved in a solvent such as water. Where the carbohydrate feed is ina non-aqueous environment, it is preferred that the carbohydrate is atleast partially hydrated. Most preferably, the carbohydrate feed isprovided in an aqueous solution. The mass ratio of water to carbohydratein the carbohydrate feed is preferably in the range of 4:1 to 1:4.Aqueous solutions of 600 or more grams per liter of certaincarbohydrates such as glucose and sucrose are sometimes commerciallyavailable. In some instances, recycle aqueous, hydrogenation solution oraliquot or separated portion thereof, may be contained in thecarbohydrate feed. Where the carbohydrate feed contains ethylene glycolor propylene glycol, the mass ratio of total ethylene glycol andpropylene glycol to carbohydrate is in the range of about 10:1 to 1:20,and sometimes in the range of about 2:1 to 1:20. It is within thepurview of this invention to add water to the carbohydrate feed prior tointroduction into the aqueous, hydrogenation medium. The carbohydratecontained in the carbohydrate feed is provided in an amount of betweenabout 120 to 700 or 800, often, about 150 to 500, grams per liter ofaqueous, hydrogenation medium. Optionally, a separate reaction zone canbe used that contains retro-aldol catalyst with an essential absence ofhydrogenation catalyst. Where a separate reaction zone containingretro-aldol catalyst is used, it is preferred that the carbohydratecontained in the carbohydrate feed to such reaction zone provide betweenabout 120 to 700 or 800, often, about 150 and 500, grams of totalcarbohydrate per liter of aqueous medium in that separate zone.

Rapid Temperature Increase

In accordance with the first broad processes of this invention thecarbohydrate feed is rapidly transitioned through the temperature zoneof 170° C. to 230° C., and preferably to a temperature of at least about240° C. The rapid heating to a temperature of at least 230° C. has beenfound by this invention to result in higher mass ratios of ethyleneglycol to propylene glycol. The aldohexose-yielding carbohydrate in acarbohydrate feed provides a product solution having a mass ratio ofethylene glycol to propylene glycol of at least about 15:1, andsometimes at least about 20:1 and as high as 30:1 or more. Where thecarbohydrate feed contains other aldose-yielding carbohydrates orketose, propylene glycol is expected to result from the retro-aldolreaction and hydrogenation and this production of ethylene glycol andpropylene glycol needs to be taken into account when determining theselectivity of conversion of aldohexose to ethylene glycol.

In some instances, the rapid heating through the temperature zone of170° C. to 230° C. has been found to provide a relatively low mass ratioof glycerin to propylene glycol. In such instances, the mass ratio ofglycerin to propylene glycol is often less than about 0.5:1. This massratio of glycerin to propylene glycol can be achieved regardless ofwhether the carbohydrate feed contains, in addition toaldohexose-yielding carbohydrate, other aldoses or ketoses. The lowproduction of glycerin as a side product is advantageous due to therelatively low market value of glycerin as compared to ethylene glycoland propylene glycol.

The carbohydrate feed can be in the presence of other chemicals duringthe heating. For instance, hydrogen for the hydrogenation may be atleast in part supplied with the carbohydrate feed. Other adjuvants, suchas pH control agents, can also be present if desired. In one embodiment,the carbohydrate feed contains retro-aldol catalyst, and in suchinstances, catalytic conversion of the aldohexose-yielding carbohydrateoccurs during the heating. The extent of conversion of thealdohexose-yielding carbohydrate during the heating will be affected,among other things, by the duration of the heating, the relativeconcentrations of the carbohydrate and the retro-aldol catalyst and theactivity of the retro-aldol catalyst.

As discussed above, the heating of the carbohydrate feed can beaccomplished in any suitable manner and one or more types of heating canbe used. All, none, or a portion of the heating of the carbohydrate feedcan occur before the carbohydrate feed is introduced into the aqueous,hydrogenation medium. For example, but not in limitation, the heating ofthe carbohydrate feed through the temperature zone of 170° C. to 230° C.can occur prior to introducing the aqueous, hydrogenation medium, andthe heated carbohydrate feed can be maintained in contact withretro-aldol catalyst to effect conversion of the aldohexose-yieldingcarbohydrate to intermediates such as glycolaldehyde and erythrose forfurther conversion to ethylene glycol. In embodiments where the heatedcarbohydrate feed is maintained in contact with retro-aldol catalystprior to being introduced into the aqueous, hydrogenation medium, theduration of such contact prior to introduction into the aqueous,hydrogenation medium is generally below about 15, preferably below about10, and in some instances below about 5, seconds. Typically, any holdtime prior to the introduction of the heated carbohydrate feed into theaqueous, hydrogenation medium is a consequence of the equipmentconfiguration such as piping distances and residence time in ancillaryequipment such as fluid distributors from the heat exchange zone intothe hydrogenation zone. As can be appreciated, turn up and turn downoperations will affect the inherent hold time.

The heat source used for the heating of the carbohydrate feed throughthe temperature zone of 170° C. to 230° C. is not critical. Forinstance, the heating can be provided by radiant or microwaveexcitation, indirect heat exchange with other process streams, or directheat exchange with a process stream also passing to the aqueous,hydrogenation medium or combinations thereof. In instances where thecarbohydrate feed through the temperature zone of 170° C. to 230° C. isheated at least in part by direct heat exchange with the aqueous,hydrogenation medium, it is generally preferred that retro-aldolcatalyst is already present in the aqueous, hydrogenation medium. Asdiscussed above, the rate of heating will be affected by heat and masstransfer parameters. It is generally desired to promote mixing of thecarbohydrate feed during the heating to facilitate both mass and heattransfer and thereby reduce the time required for the carbohydrate feedto fully pass through this temperature zone. This mixing can be affectedin any suitable manner including, but not limited to, mechanical andstationary mixing and rapid diffusional mixing. The thoroughness of themixing also can affect the mass transfer of reactants, intermediates,catalysts and products and thus affect selectivities of conversion toethylene glycol and the rate of formation of side products.

A particularly useful stream for direct heat exchange with thecarbohydrate feed is withdrawn product solution (recycle). If a solubleretro-aldol catalyst is used in the aqueous, hydrogenation medium therecycle provides for a substantial return of the retro-aldol catalyst tothe reaction system. The recycle can be at a temperature of at leastabout 180° C., say, at a temperature in the range of about 230° C. to300° C. The mass ratio of recycle to carbohydrate feed will depend uponthe relative temperatures of the two streams and the sought combinedtemperature. Often where a recycle is used, the mass ratio of recycle tocarbohydrate feed is in the range of about 1:1 to 100:1. The recycle maybe an aliquot portion of the withdrawn product solution, or may besubjected to unit operations to separate one or more components fromrecycle stream, such as, but not in limitation, degassing to removehydrogen and filtration to remove, e.g., any entrained heterogeneouscatalyst. Where the product solution is degassed to recover at least aportion of the hydrogen, the recycle is frequently an aliquot portion ofthe degassed product solution. One or more components can be added tothe recycle prior to combination with the carbohydrate feed in thedirect heat exchange in operation. These components include, but are notlimited to, retro-aldol catalyst, pH control agents, and hydrogen. Byusing a recycle of withdrawn product solution, the combined carbohydratefeed and recycle can contain unreacted aldose-yielding carbohydrate,intermediates to ethylene glycol, and ethylene glycol. Where acarbohydrate feed is used which is not in aqueous solution, e.g., is asolid or is a melt, the recycle provides water to dissolve thecarbohydrate and to stabilize the carbohydrate from caramelization.

The Conversion Process

The processes of this invention provide high conversion efficiency ofaldose-yielding carbohydrate to ethylene glycol through retro-aldol andhydrogenation reactions. In the processes, the carbohydrate feed isintroduced into an aqueous, hydrogenation medium that containsretro-aldol catalyst, hydrogen and hydrogenation catalyst. Thecarbohydrate feed may or may not have been subjected to retro-aldolconditions prior to being introduced into the aqueous, hydrogenationmedium, and the carbohydrate feed may or may not have been heatedthrough the temperature zone of 170° C. to 230° C. upon contacting theaqueous, hydrogenation medium. Thus, in some instances the retro-aldolreactions may not occur until the carbohydrate feed is introduced intothe aqueous, hydrogenation medium, and in other instances, theretro-aldol reactions may have at least partially occurred prior to theintroduction of the carbohydrate feed into the aqueous, hydrogenationmedium. It is generally preferred to quickly disperse the carbohydratefeed in the aqueous, hydrogenation medium especially where the aqueous,hydrogenation medium is used to provide direct heat exchange to thecarbohydrate feed. This dispersion can be achieved by any suitableprocedure including, but not limited to, the use of mechanical andstationary mixers and rapid diffusional mixing.

The preferred temperatures for retro-aldol reactions are typicallybetween about 230° C. and 300° C., and more preferably between about240° C. and 280° C., although retro-aldol reactions can occur at lowertemperatures, e.g., as low as 90° C. or 150° C. The pressures (gauge)are typically in the range of about 15 to 200 bar (1500 to 20,000 kPa),say, between about 25 and 150 bar (2500 and 15000 kPa). Retro-aldolreaction conditions include the presence of retro-aldol catalyst. Aretro-aldol catalyst is a catalyst that catalyzes the retro-aldolreaction. Examples of compounds that can provide retro-aldol catalystinclude, but are not limited to, heterogeneous and homogeneouscatalysts, including catalyst supported on a carrier, comprisingtungsten and its oxides, sulfates, phosphides, nitrides, carbides,halides and the like. Tungsten carbide, soluble phosphotungstens,tungsten oxides supported on zirconia, alumina and alumina-silica arealso included. Preferred catalysts are provided by soluble tungstencompounds, such as ammonium metatungstate. Other forms of solubletungstates, such as ammonium paratungstate, partially neutralizedtungstic acid and sodium metatungstate. Without wishing to be limited totheory, the species that exhibit the catalytic activity may or may notbe the same as the soluble tungsten compounds introduced as a catalyst.Rather, a catalytically active species may be formed in the course ofthe retro-aldol reaction. The concentration of retro-aldol catalyst usedmay vary widely and will depend upon the activity of the catalyst andthe other conditions of the retro-aldol reaction such as acidity,temperature and concentrations of carbohydrate. Typically, theretro-aldol catalyst is provided in an amount to provide between about0.05 and 100, say, between about 0.1 and 50, grams of tungstencalculated as the elemental metal per liter of aqueous, hydrogenationmedium. The retro-aldol catalyst can be added as a mixture with thecarbohydrate feed or as a separate feed to the aqueous, hydrogenationmedium or both.

Where the carbohydrate feed is subjected to retro-aldol conditions priorto being introduced into the aqueous, hydrogenation medium, preferablythe introduction into the aqueous, hydrogenation medium occurs in lessthan one, sometimes less than about 0.5, and in some instances less thanabout 0.1, minute from the commencement of subjecting the carbohydratefeed to the retro-aldol conditions. Often, at least about 10, preferablyat least about 20, percent of the aldose-yielding carbohydrate in thecarbohydrate feed is remaining upon introduction into the aqueous,hydrogenation medium. By continuing the retro-aldol conversions of thecarbohydrate in the aqueous, hydrogenation medium, the duration of timebetween the retro-aldol conversion of the aldose to the commencement ofcontact with the hydrogenation catalyst is reduced.

Typically where aldose-yielding carbohydrate is present in the aqueous,hydrogenation medium the aqueous, hydrogenation medium is maintained ata temperature of at least about 230° C. until substantially allaldose-yielding carbohydrate is reacted. Thereafter, if desired, thetemperature of the aqueous, hydrogenation medium can be reduced.However, the hydrogenation proceeds rapidly at these highertemperatures. Thus the temperatures for hydrogenation reactions arefrequently between about 230° C. and 300° C., say, between about 235° or240° C. and 280° C. The pressures (gauge) are typically in the range ofabout 15 to 200 bar (1500 to 20,000 kPa), say, between about 25 and 150bar (2500 to 15,000 kPa). The hydrogenation reactions require thepresence of hydrogen as well as hydrogenation catalyst. Due to the lowsolubility of hydrogen in aqueous solutions, the concentration ofhydrogen in the aqueous, hydrogenation medium will primarily bedetermined by the partial pressure of hydrogen in the reactor. The pH ofthe aqueous, hydrogenation medium is often at least about 3, say,between about 3.5 and 8, and in some instances between about 4 and 7.5.

The hydrogenation is conducted in the presence of a hydrogenationcatalyst. The hydrogenation catalyst can also be referred to as reducingmetal catalysts and are catalysts for the reduction of carbonyls.Frequently the hydrogenation catalyst is a heterogeneous catalyst. Itcan be deployed in any suitable manner, including, but not limited to,fixed bed, fluidized bed, trickle bed, moving bed, slurry bed, andstructured bed. Nickel, palladium and platinum are among the more widelyused reducing metal catalysts. However many reducing catalysts will workin this application. The reducing catalyst can be chosen from a widevariety of supported transition metal catalysts. Nickel, Pt, Pd andruthenium as the primary reducing metal components are well known fortheir ability to reduce carbonyls. One particularly favored catalyst forthe reducing catalyst in this process is a Ni—Re catalyst supported onsilica alumina. A similar version of Ni/Re or Ni/Ir can be used withgood selectivity for the conversion of the formed glycolaldehyde toethylene glycol. Nickel-rhenium is a preferred reducing metal catalystand may be supported on alumina-silica, silica or other supports.Supported Ni—Re catalysts with B as a promoter are useful. Frequently ina slurry reactor, the hydrogenation catalyst is provided in an amount ofbetween about 0.1 to 100, and more often, between about 0.5 or 1 and 50,grams per liter of aqueous, hydrogenation medium and in a packed bedreactor the hydrogenation catalyst comprises about 20 to 80 volumepercent of the reactor.

Typically the retro-aldol reaction proceeds more quickly than thehydrogenation reaction and consequently the residence time of thecarbohydrate feed in the hydrogenation reactor is selected to reflectthe sought degree of hydrogenation. In some instances, the weight hourlyspace velocity is between about 0.01 and 20, and frequently betweenabout 0.02 and 5, hr⁻¹ based upon total carbohydrate in the feed. Insome instances it is desired to maintain the aqueous, hydrogenationmedium is well dispersed to assure relatively uniform concentrations ofintermediates to ethylene glycol therein.

The retro-aldol and hydrogenation environment can result in undesiredreactions. See, for instance, the Green Chem. article cited in theBackground section. Table 1 on page 697 and Table 4 on page 700 of thearticle reports the product composition from subjecting various aldosesto retro-aldol and hydrogenation conditions. The primary side productsthey report include sorbitol, erythritol, propylene glycol and glycerol.Not only do these side products represent a loss of conversionefficiency to ethylene glycol but also side products such as glycerolhave an economic value lower than that of ethylene glycol. The sideproducts can further be reacted under these conditions, and ethyleneglycol degraded as reported in their Table 3. The formation of sideproducts can in part be attributed to the generation of reactive speciesas intermediates, especially glycolaldehyde and erythrose.

In the processes of this invention, the combination of reactionconditions (e.g., temperature, hydrogen partial pressure, concentrationof catalysts, hydraulic distribution, and residence time) are sufficientto convert at least about 95, often at least about 98, mass percent andsometimes essentially all of the aldohexose-yielding carbohydrate. It iswell within the skill of the artisan having the benefit of thedisclosure herein to determine the set or sets of conditions that willprovide the sought conversion of the aldohexose-yielding carbohydrate.The processes of this invention also use a combination of conditionsthat can provide a conversion efficiency of the aldohexose to ethyleneglycol of at least about 60, preferably at least about 70, and morepreferably at least about 75, mass percent. The conversion efficiency toethylene glycol is determined as the mass of ethylene glycol in theproduct solution as a percentage of the mass of aldohexose-yieldingcarbohydrate in the carbohydrate feed. Where the carbohydrate feedcontains other carbohydrates, the contributions of the other feed to theethylene glycol content are taken into account.

Without wishing to be limited by theory, it is believed that theformation of intermediates by the retro-aldol reaction needs to be inclose time proximity to the hydrogenation of those intermediates toethylene glycol such that they are hydrogenated before a significantamount of the intermediates can be consumed in competitive reactions.Accordingly, a balance between the retro-aldol catalyst andhydrogenation catalyst can be ascertained for a given retro-aldolcatalyst and a given hydrogenation catalyst under the conditions of thereaction in order to achieve a high conversion efficiency to theethylene glycol. Additionally it is believed that the rapid heating ofthe carbohydrate feed provides the feed at temperatures where theretro-aldol rate of reaction can be more easily matched to thehydrogenation rate of reaction.

It is believed that the ratio of the retro-aldol catalyst tohydrogenation catalyst can also serve to attenuate the production ofhexitol by both minimizing the presence of carbohydrate and providingconcentrations of intermediates to ethylene glycol that preferentiallygo to the active hydrogenation sites. One mode of operation of processesin accordance to certain embodiments uses a homogeneous retro-aldolcatalyst and a heterogeneous hydrogenation catalyst such thatretro-aldol catalyst can be physically located proximate to thehydrogenation catalyst. The intermediates, being smaller molecules,diffuse more rapidly to the catalyst sites than the larger carbohydratemolecules, and with the limited solubility of hydrogen in the aqueous,hydrogenation medium, hydrogen mass transfer rates to the hydrogenationcatalyst are believed to modulate the hydrogenation reaction. Preferablythe mass ratio of ethylene glycol to hexitol in the product solution isgreater than about 10:1, and in some instances is greater than about20:1 or 25:1 or even greater than about 40:1 or 50:1. As discussedabove, providing total carbohydrate in the carbohydrate feed in anamount of about 120 to 700 or 800, or 150 to 500, grams per liter ofaqueous, hydrogenation medium can serve to attenuate the rate ofproduction of 1,2-butanediol.

Determining a suitable ratio of retro-aldol catalyst to hydrogenationcatalyst is within the skill of the artisan having the benefit of thedisclosures herein. The ratio will depend on, among other things, therelative activities of the two catalysts under steady-state conditions.The relative activities are influenced by the intrinsic activity of thecatalysts, and the physical configuration of the catalysts. Hence, theratios of these catalysts can vary widely over a range of retro-aldolcatalysts and hydrogenation catalysts. However, for a given retro-aldolcatalyst and hydrogenation catalyst, desirable ratios can be determined.If a retro-aldol reaction zone having an essential absence ofhydrogenation catalyst is used, as taught by Schreck, et al., in U.S.Published Pat. Appl. 2015/0329449, the conditions, including, but notlimited to, hydraulic residence time and retro-aldol catalystconcentration, can be adjusted to achieve the sought conversionefficiencies to ethylene glycol and mass ratios of ethylene glycol tosorbitol. If desired, the reaction zone containing the hydrogenationcatalyst can have differing ratios of retro-aldol catalyst tohydrogenation catalyst. For example, in a continuous, stirred tankreactor using a homogeneous retro-aldol catalyst and a heterogeneoushydrogenation catalyst and the carbohydrate feed being introduced at orimmediately below the surface of the aqueous, hydrogenation medium, therate of stirring may be such that a density gradient for thehydrogenation catalyst exists. The lesser concentration of hydrogenationcatalyst at the top of the aqueous, hydrogenation medium enablescarbohydrates to be subjected to the retro-aldol reaction prior tosignificant amounts of hydrogenation occurring.

Post Reaction Processing

A product solution is withdrawn from the reaction zone continuously orintermittently. Following the reactor, a part of the withdrawn productsolution may be separated for recycle back to the front of the processas described above. Preferably, at least a portion of the retro-aldolcatalyst is recycled or recovered from the withdrawn product solutionfor recycle. The withdrawn product solution can be depressurized withthe gases being captured for recovery of the hydrogen and removal ofunwanted gaseous by-products such as methane and carbon dioxide.

Upon cooling, less soluble portions of catalysts that are solubilizedfrom the bed or that are fed to the reactor are removed at the reducedtemperature and the remaining liquid is transferred to the recoveryportion of the process. Depending upon catalyst stability andsolubility, it is possible to take the degassed reactor effluent torecovery where a portion of the volatile products are recovered and theheavy bottoms are treated for, e.g., recovery of the tungsten catalystfor reuse in the reactor.

In recovery the low boiling components such as ethanol and methanol areremoved via distillation. Water is also removed via distillationfollowed by recovery of propylene glycol and ethylene glycol. It istypical in the manufacture of ethylene glycol to use multi-effectedevaporators to minimize energy usage in the recovery of the ethyleneglycol.

It is likely that separation of the ethylene glycol from the propyleneglycol or other close boiling glycols will require an additional, moresophisticated separation technology. Simulated moving bed technology isone such option that can be used. The choice is dependent on the qualityof the product that is required by the desired end use for the product.

Drawing

Reference is made to the drawing which is provided to facilitate theunderstanding invention but is not intended to be in limitation of theinvention. The drawing is a schematic depiction of an apparatusgenerally designated as 100 suitable for practicing the instantlydisclosed processes. The drawing omits minor equipment such as pumps,compressors, valves, instruments and other devices the placement ofwhich and operation thereof are well known to those practiced inchemical engineering. The drawing also omits ancillary unit operations.

A carbohydrate feed is provided by line 102. The carbohydrate feed maybe a solid or liquid including in a solution with water. For purposes ofdiscussion, the carbohydrate feed is an aqueous glucose solutioncontaining about 71 volume percent carbohydrate. A retro-aldol catalystis provided via line 104. The addition of a retro-aldol catalyst isoptional at this point in the process. For purposes of discussion, theretro-aldol catalyst is ammonium metatungstate in an aqueous solution,and the ammonium metatungstate is provided in an amount sufficient tohave a concentration of ammonium metatungstate of about 10 grams perliter.

The carbohydrate feed is then combined with a warmer, recycle stream ofwithdrawn product solution as will be described later. This combinationaffects a direct heat exchange to increase the temperature of thecarbohydrate feed and provide a combined stream. This combined streamthen passes via line 106 to distributor 108 and reactor 110. Distributor108 may be of any suitable design. For purposes of discussion,distributor 108 is spray head that distributes the combined stream asfine droplets over the surface of the aqueous, hydrogenation medium 112in reactor 110. Reactor 110 contains agitator 114 to provide mechanicalmixing of the aqueous, hydrogenation medium 112. This mechanical mixingassist in dispersing the fine droplets of the combined stream within theaqueous, hydrogenation medium to further enhance the rate that thecombined stream is brought to the temperature of the aqueous,hydrogenation medium. It also assists in the mass transfer ofintermediates from the retro-aldol reaction to the hydrogenationcatalyst. Reactor 110 also contains a particulate, heterogeneoushydrogenation catalyst, by way of example, nickel/rhenium/boronhydrogenation catalyst on silica support, which catalyst is dispersed inthe aqueous, hydrogenation medium by the mechanical mixing.

Hydrogen is supplied to reactor 110 through line 116. The hydrogen maybe supplied through a nozzle to provide small bubbles of hydrogen tofacilitate mass transfer of hydrogen into the aqueous, hydrogenationmedium. If desired, an additional retro-aldol catalyst and otheradjuvants may be supplied to the reactor the line 118.

Aqueous, hydrogenation medium is withdrawn from reactor 110 via line 120as the product solution. As shown, a portion of the product solution ispassed via line 122 to line 106 as recycle to be combined with thecarbohydrate feed 102. This recycle will contain homogeneous retro-aldolcatalyst. Optionally, the recycle stream in line 122 can be heated inindirect heat exchanger 124 to enable the combined stream in line 106 tohave a higher temperature.

The following examples are provided to further illustrate the inventionand are not in limitation of the invention. All parts and percentagesare by mass unless otherwise stated.

EXAMPLES 1 TO 20

The following general procedure is used in examples 1 to 20.

A 300 ml Hastelloy C Parr reactor is equipped with an agitator and oneor two feed supply lines and a dip tube attached to a sample bomb. Theend of the dip tube is positioned such that about 100 milliliters ofsolution would remain in the reactor. The reactor is charged withheterogeneous hydrogenation catalyst and an aqueous solution oftungsten-containing retro-aldol catalyst as specified in Table I. Thecharge is approximately 170 milliliters of aqueous solution. The reactoris then sealed and purged to remove oxygen. Purging is accomplished bythree cycles of pressurizing the reactor to 50 psig (345 kPa gauge) withnitrogen, then venting to atmospheric pressure. The liquid level in thereactor is reduced to about 100 milliliters by draining through the diptube. An additional three cycles of purging, while the aqueous solutionis being stirred, are conducted using hydrogen to reduce theconcentration of nitrogen, then venting to atmospheric pressure.

Stirring is commenced and is at a rate sufficient to maintain theheterogeneous hydrogenation catalyst in a slurry dispersion. The reactoris heated to reaction temperature as specified in Table I andpressurized under hydrogen to 10700 kilopascals gauge. When the reactorreaches operating temperature and pressure, feed of glucose solution isinitiated and maintained at a constant rate for the duration of the runas is specified in Table I. Retro-aldol catalyst is continuously addedat a constant rate for the duration of the run. Near-continuousoperation is achieved by draining the reactor at regular intervals to aconstant liquid level determined by the position of the dip tube. Afilter attached to the end of the dip tube ensures all heterogeneouscatalyst particles are retained inside the reactor. Every 10 to 15minutes, the reactor pressure is adjusted to 10700 kilopascals by eitheradding hydrogen or venting.

A sample of the aqueous medium is taken during operation through the diptube and sample bomb and cooled to room temperature. Typically thesample is taken after about 4 hours of operation although the sample inExample 16 is taken after about 1.6 hours, in Example 20, 2 hours, andin Example 3, after about 2 hours. The samples are analyzed by highpressure liquid chromatography (HPLC) and gas chromatography (GC). TheHPLC is equipped with a refractive index detector and uses a Hi-Plex Hresin column available from Agilent Technologies, Santa Clara, Calif.The GC analysis is performed with an HP 5890 GC (Agilent Technologies,Santa Clara, Calif.) using a flame ionization detector with 25:1 splitinjection. A J&W DB-WAX 30 m×0.32 mm×0.5 micron capillary column(Agilent Technologies, Santa Clara, Calif.) is used.

Except as otherwise stated, ammonium metatungstate is used as theprecursor to the catalytically-active retro-aldol catalyst species.Unless otherwise stated, the hydrogenation catalyst is a nickel, rheniumand boron on silica alumina catalyst prepared using the procedure setforth at column 8, line 62, to column 9, line 27, of U.S. Pat. No.6,534,441. The silica alumina support is a 3 millimeter extrudate andhas a surface area of about 125 square meters per gram and a pore volumeof about 0.7 to 0.9 milliliters per gram. The catalysts contain about6.8 mass percent nickel, unless otherwise stated, and the mass ratio ofthe atoms of nickel:rhenium:boron is about 5:1.3:1.

In the examples, glucose is used as the carbohydrate feed and isprovided at a concentration of about 32.4 mass percent in an aqueoussolution except as otherwise noted. The glucose-containing feedcontains, when a single feed supply line is use, retro-aldol catalyst.Where two feed supply lines, one is used for supplying the carbohydratefeed and the other is used to supply an aqueous solution of retro-aldolcatalyst. The dimensions of the supply lines differ in inside diameterand length as well as positioning in the reactor. The supply lines thatare used have either a ⅛ inch (3.2 millimeter) outside diameter or a1/16 inch (1.6 millimeter) outside diameter. The length and diameter ofthe supply lines and their positioning affect the rate of heating of thecarbohydrate feed as does the rate of flow through the supply lines. Ata feed rate of 1 milliliter per minute, the residence time per 2.5centimeters of length is about 0.4 second for a 1/16 inch supply lineand about 3.7 seconds per 2.5 centimeters of length of a ⅛ inch supplyline. Where two supply tubes are used, the feed rate in Table I is thesum of both feeds.

Comparative examples are designated by the abbreviation “comp” in TableI.

Table II summarizes the product compositions in each of the samples.

TABLE I R-A Cat., Hydrog. Temp., Feed rate, Example mass % Cat., grams °C. ml/min. Feed system  1 1.0 6 245 1 Single 1/16 inch supply line 5centimeters in length  2 1.0 4 245 1 Single 1/16 inch supply line 5centimeters in length  3 1.0 6 245 2 Single 1/16 inch supply line 5centimeters in length  4 (comp)^(a) 1.0 6 245 1 Single 1/16 inch supplyline 5 centimeters in length  5 0.25 6 245 1 Single 1/16 inch supplyline 5 centimeters in length  6^(b) 1 6 245 1 Single 1/16 inch supplyline 5 centimeters in length  7 0.25 6 255 1 Single 1/16 inch supplyline 5 centimeters in length  8 0.1 6 245 1 Single ⅛ inch supply line 10centimeters in length  9^(c) 1 6 245 1 Single ⅛ inch supply line 10centimeters in length 10 (comp)^(d) 0.1 6 245 1 Single ⅛ inch supplyline 10 centimeters in length 11 1 6 245 1 Two supply lines, each 1/16inch and 5 centimeters in length 12 1 6 245 1 Two supply lines, forcarbohydrate feed, an insulated 1/16 inch 5 centimeters line, and forretro-aldol, 1/16 inch 10 centimeters line extending below the aqueoussolution level 13 1 6 245 1 Two supply lines, each ⅛ inch by 10centimeters 14 (comp) 0.5 6 225 1 Single ⅛ inch supply line 5centimeters in length 15 1  2^(e) 245 1 Single ⅛ inch supply line 5centimeters in length 16 0.5^(f)  2^(e) 245 2.5 Single ⅛ inch supplyline 5 centimeters in length 17 1 6 245 1 Single 1/16 inch supply line 5centimeters in length (insulated to minimize heating of feed) 18^(g) 1 6245 0.9 Single ⅛ inch supply line 5 centimeters in length 19^(g) 1.5 6245 0.9 Single ⅛ inch supply line 5 centimeters in length 20 ^(h) 6 2452.5 Single ⅛ inch supply line 5 centimeters in length ^(a)pressure is5500 kilopascals gauge ^(b)feed contains 31.6 mass percent glucose, 37mass percent propylene glycol and water ^(c)feed contains 32.4 masspercent glucose, 30 mass percent ethylene glycol and water ^(d)feedcontains 32.4 mass percent glucose, 30 mass percent ethylene glycol andwater ^(e)hydrogenation catalyst is ruthenium on carbon at a 5% loading(product number 206180) available from Sigma-Aldrich Chemical Company,St. Louis, Mo, U.S.A. ^(f)sodium metatungstate is used ^(g)feed contains50 mass percent glucose ^(h)0.5 mass percent sodium metatungstate and0.6 mass percent sodium tungstate

TABLE II Conversion Conversion of Efficiency of 1,2- Carbohydrate,Aldose to EG, EG:PG, EG:Hexitol^(a) Glycerin:PG Butanediol, Example mass% mass % mass mass mass mass %  1 100 79 29:1 150:1 0.3:1 1.3  2 100 8529:1 100:1 0.3:1 1.3  3 100 70 17:1  58:1 0.4:1 1.9  4 (comp) 100 3710:1 370:1 0.3:1 2.2  5 100 77 24:1  27:1 0.6:1 1.3  6 100 86 n/a^(b)172:1 n/a^(b) 1.4  7 100 64 20:1 320:1 0.3:1 1.7  8 100 68 15:1  26:10.6:1 2.0  9 100 72 15:1 180:1 0.2:1 3.0 10 (comp) 100 63 15:1  27:10.6:1 2.0 11 100 80 28:1 160:1 0.2:1 1.6 12 100 83 34:1 410:1 0.2:1 1.113 100 73 16:1 240:1 0.2:1 1.7 14 (comp) 100 54  9:1  11:1 0.6:1 4 15100 76 24:1 ^(c) 0.3:1 1.7 16 100 69 17:1 110:1 0.4:1 2.5 17 100 83 26:1410:1 0.2:1 1.9 18 100 79 17:1 ^(c) 0.2:1 3.2 19 100 82 20:1 820:1 0.6:12.6 20 100 68 16:1  92:1 0.5:1 1.5 ^(a)hexitol approximates sorbitolcontent ^(b)not calculated due to PG in feed stream ^(c) no hexitol isdetected

It is claimed:
 1. A highly selectivity, continuous process forconverting aldohexose in an aldohexose-yielding carbohydrate feed toethylene glycol by retro-aldol conversion of the aldohexose toglycolaldehyde and hydrogenation of glycolaldehyde to ethylene glycol,comprising: a. continuously or intermittently passing saidcarbohydrate-containing feed into a reaction zone having an aqueous,hydrogenation medium containing homogeneous retro-aldol catalyst,hydrogen and hydrogenation catalyst; b. providing at least one pHcontrol agent to the hydrogenation zone to maintain the hydrogenationmedium at a pH between about 3 and 8; c. maintaining the aqueous,hydrogenation medium in the reaction zone at hydrogenation conditions toprovide a product solution comprising ethylene glycol, saidhydrogenation conditions being at retro-aldol conditions of atemperature between about 230° C. and 300° C. and providing a ratio ofretro-aldol catalyst to hydrogenation catalyst, and a hydrogen partialpressure that, in combination are sufficient to provide a conversionefficiency of said aldohexose to ethylene glycol of at least 60 percent,wherein said carbohydrate feed is at least partially hydrated and isunder a pressure sufficient to maintain partial hydration; wherein saidcarbohydrate feed is below a temperature of about 170° C.; and whereinsaid carbohydrate-containing feed is rapidly heated to above 230° C.;and d. continuously or intermittently withdrawing said product solutionfrom the reaction zone, which withdrawn product contains retro-aldolcatalyst.
 2. The process of claim 1 wherein the aldohexose-yieldingcarbohydrate being passed to the aqueous medium is in an aqueoussolution.
 3. The process of claim 2 wherein the aqueous solutioncontains retro-aldol catalyst.
 4. The process of claim 1 wherein theheating of the carbohydrate feed from below 170° C. to above 230° C. isat least in part by direct heat exchange by admixing the carbohydratefeed with a warmer fluid.
 5. The process of claim 1 wherein thehydrogenation medium is at a pH between about 3.5 and
 8. 6. The processof claim 1 wherein the pH control agent comprises buffer.
 7. The processof claim 1 wherein the pH control agent comprises base.
 8. The processof claim 1 wherein at least a portion of the retro-aldol catalystcontained in the withdrawn product solution is recycled to the reactionzone.
 9. The process of claim 1 wherein the pH control agent is added tothe recycle.
 10. The process of claim 9 wherein the pH control agentcomprises buffer.
 11. The process of claim 9 wherein the pH controlagent comprises base.
 12. The process of claim 1 wherein thehydrogenation medium is at a pH between about 3.5 and
 8. 13. The processof claim 1 wherein the aldohexose-yielding carbohydrate being passed tothe aqueous medium is in an aqueous solution containing retro-aldolcatalyst and having an essential absence of hydrogenation catalyst. 14.The process of claim 1 wherein the wherein the aldohexose-yieldingcarbohydrate being passed to the aqueous medium is in an aqueoussolution and the aqueous solution contains ethylene glycol.
 15. Theprocess of claim 14 wherein the aqueous solution contains retro-aldolcatalyst and has an essential absence of hydrogenation catalyst.
 16. Theprocess of claim 1 wherein the aqueous hydrogenation medium containssodium tungstate.